This invention relates to DC to DC converters and, in particular, to voltage balance control circuits associated with these converters.
The prior art DC to DC converter circuits implement the function of converting an AC signal to a DC signal by first converting the AC signal to a first DC signal that is then inverted to a high frequency, and generally pulse width modulated, AC signal which is transformer coupled to a second AC to DC converter that converts the high frequency AC signal to the output DC signal. A feedback path monitors the output voltage level and provides pulse width modulated control signals for controlling the inverter.
FIG. 1 is a diagram of the prior art circuits and shows an input AC to DC converter 7, the output of which is applied to an inverter 13. The inverter 13 inverts the DC signal to a high frequency pulse width modulated AC signal which is coupled by the transformer 15 to the output AC to DC converter 17. The inverter controller 19 senses the DC output signal from the output AC to DC converter 17 and compares the sensed DC output signal with an internally generated reference signal. The result of the comparison is an error signal that is used to generate the pulse width modulated control signals for controlling the operation of the inverter 13. The input AC to DC converter 7 is a split capacitor half bridge converter and has associated with it a full wave bridge rectifier, the output of which is applied to two series connected capacitors for filtering.
The basic problem encountered with the converter 7 is the occurrence of a voltage difference across the capicitors. Any imbalance in the power circuits, especially in the reflected impedances created by the transformer 15 on each half cycle, can force either of the capacitors to have a different voltage than the other capacitor. Although the control loop, that includes the inverter controller 19, has an inherent tendency to balance the loading on the capacitors 16 and 18, the imbalanced margin is limited by the transformer 15 saturating if the high frequency voltages should happen to be imbalanced. Although the saturating of the transformer 15 has a balancing effect, there can still be undesirable imbalanced voltages on either capacitor which will result in the inverter 13 switching high currents.